The present invention relates generally to methods for providing high power, depolarized superluminescent diodes (SLDs) and, more particularly, to providing high power, depolarized SLDs using one or more polarization sensitive semiconductor optical amplifiers (SOAs) in conjunction with a polarization beam combiner.
Optical technologies have evolved rapidly over the last two decades. Optical information communication technologies, for example, have evolved as the technology of choice for backbone information communication systems due to, among other things, their ability to provide large bandwidth, fast transmission speeds and high channel quality. Optical sensing and imaging technologies have also found widespread acceptance in applications ranging from aerospace to medical. Various types of optical light sources have been used in optical technologies. Three such semiconductor-based sources are the light emitting diode (LED), the laser diode (LD) and the superluminescent diode (SLD). It should be noted that the SLD is also often referred to as a superluminescent LED (SLED). LEDs produce light through spontaneous emission of photons when a current is passed through them. Spontaneous emission refers to the random generation of photons within an active layer of the LED, these photons are emitted in random directions. This process causes the light output from LEDs to be incoherent, have a broad spectral width and have a wide output beam pattern. Laser diodes, on the other hand, produce light through stimulated emission when a current is passed through them. In the LD, photons initially produced by spontaneous emission interact with the laser material to produce additional photons. This process occurs within the active area of the LD referred to as the laser cavity. The emission process and the physical characteristics of the diode cause the light output to be coherent, have a narrow spectral width and have a comparatively narrow output beam pattern. The output power from a LD is typically much higher than from an LED. SLDs produce light through a combination of both spontaneous emission and stimulated emission when a current is passed through them. Spontaneous emission of photons is amplified within the SLD active layer to generate “superluminescent” or amplified spontaneous emission (ASE) output power. In this manner the SLD has low-coherence and broad spectral width similar to an LED, yet high output power and narrow output beam pattern (spatial coherence) similar to a LD. Because of this desirable combination of features, SLDs are extremely useful optical sources for a range of applications including, for example, optical coherence tomography, optical fiber gyroscopes and optical fiber sensor systems.
The optical output power from an SLD is typically linearly polarized. There are many applications such as in optical imaging and optical fiber sensors where a depolarized broadband optical source is desired to reduce or eliminate unwanted polarization sensitivity. Several options currently exist for achieving a depolarized broadband optical source. Optical fiber amplifiers based on either Erbium or Ytterbium, for example, provide a depolarized output signal since the gain medium is intrinsically polarization independent. However, fiber amplifiers are larger and more expensive than semiconductor-based SLD sources. Also, fiber amplifiers operate over a much more limited range of wavelengths compared to SLDs. Alternatively, the degree of polarization (DOP) of the SLD output can be reduced using special depolarizing optical elements such as a wedge depolarizer, Lyot depolarizer or various types of time-domain polarization scramblers. However, the efficacy of these depolarizing approaches depends on the details of the application; often there are fundamental incompatibilities between the physics of the depolarization mechanism and the optical system design. Therefore an SLD optical source that is intrinsically depolarized is needed.
A semiconductor optical amplifier (SOA) can potentially be used as a depolarized SLD. SOAs are compact semiconductor devices that can be made to operate over a wide wavelength range, for example to generate optically amplified signals in optical transmission systems. An SOA receives an input signal and amplifies the signal to a level which is typically on the order of 10 to 30 dB above the input signal strength on a single pass. One of the major challenges is to design and manufacture the SOA to amplify all polarization states with equal gain, that is, to be a “polarization insensitive” amplifier. When no input signal is applied to an SOA, light is still generated at both the input and output ports due to ASE. This ASE output has a spectrally broad emission profile that can be used as a superluminescent source. However, the ASE output from an SOA is typically limited in output power due to the design tradeoffs required to equalize the gain for both transverse electric (TE) and transverse magnetic (TM) polarization states. Also, the degree of polarization of the ASE output power is not sufficiently low due to finite differences between the TE and TM gain. That is, the SOA gain is not truly polarization insensitive especially not over a large optical bandwidth.
Although polarization insensitive SOAs have been developed, these devices can be complex and/or expensive to manufacture. Accordingly, there is a need to increase the ASE output power, and depolarize the ASE output power of an SOA (or reduce the degree of polarization) for use as a SLD.